BEIRUT — An Islamic State convoy stranded in the Syrian desert for five days has split up, and some fighters may have found their way into Iraq despite the U.S. military’s determination to stop them from reaching ­militant-controlled territory, ­Syrian activists and Iraqi officials said Sunday.

Conflicting reports and claims put the 17 buses that made up the original convoy in a variety of locations, illustrating the difficulty of establishing with any certainty events in the remote desert war zone spanning Iraq and Syria.

The buses set out in a convoy from western Syria on Tuesday under the terms of a deal brokered by the Lebanese Shiite Hezbollah movement to relocate the fighters to the Islamic State-controlled town of Bukamal on the Iraqi border in return for the bodies of Lebanese army, Hezbollah and Iranian soldiers.

The convoy became the center of a regionwide controversy over whether such deals are acceptable, with the United States and its allies trading accusations with the Iranian-backed Hezbollah and its allies over who is doing more to fight terrorism.

Iraq’s government expressed outrage at the relocation, which would have enabled the 300 Islamic State fighters in the convoy to reinforce militant positions in Iraq. The U.S. military vowed to prevent them from doing so and on Wednesday blocked the convoy’s path by bombing the desert road ahead of it.

At least some of the buses have since been stranded in the desert between Syrian government and Islamic State lines, with U.S. warplanes circling overhead to deter any further attempts to reach Islamic State territory.

The U.S. military said Sunday that six of the buses had crossed back into Syrian government-held territory and headed toward the town of Palmyra, leaving 11 buses stuck in the desert. The whereabouts of the six buses that headed to Palmyra were not clear.

Hezbollah, meanwhile, said that four of the buses reached territory controlled by the Islamic State, in fulfillment of the Hezbollah deal, and that six were stuck in the desert. It did not say what had happened to the other seven buses.

According to Syrians in the area and Iraqi officials, however, all or most of the original fighters who set out on the convoy have gotten off the buses and made their way to Iraq, using back roads to bypass the route bombed by U.S. warplanes.

Omar Abu Layla, who heads an activist network called Deir al-Zour 24, said the fighters traveled on foot to meet up with Islamic State fighters nearby and have been transported to two western Iraqi towns, Rawah and Aana. He cited the accounts of two reporters in his network who live in the area.

Two Iraqi officials said they believed all of the fighters and their families had arrived in Rawah in recent days. Residents of Rawah told Mohammed Karbouli, a member of Iraq’s parliamentary committee on defense and security, that hundreds of Islamic State fighters from Syria showed up in the town Friday and that they were apparently from the convoy.

“That deal was a big mistake, and it only harms Iraq,” he said.

Asmaa al-Ani, a member of the local council in Iraq’s Anbar province, said residents of Rawah told her that about 700 Islamic State fighters and their families had arrived and had taken up residence in empty homes. “These reinforcements will have a negative impact on the military situation for the coming operations,” she said, referring to the Iraqi army’s plans to recapture the area, one of the last remaining pockets in Iraq controlled by the Islamic State.

The claims left it unclear who, if anyone, may still be aboard the buses stranded in the desert.

On Monday, the U.S. military declined to comment on whether any fighters may have reached Iraq but said that it is continuing to monitor the buses. In an emailed statement, a spokesman pointed out that the U.S.-led coalition has struck about 85 individual Islamic State fighters and about 40 vehicles in the vicinity of the convoy in recent days. They included a tank, an artillery system and transport vehicles that were “seeking to facilitate the movement of ISIS fighters to the border area of our Iraqi partners.”

Iran’s Foreign Ministry on Sunday condemned the U.S. military’s surveillance of the buses, saying it is endangering the lives of pregnant women on board because they lack access to food and water. The U.S. military said it will not prevent supplies from reaching the buses.

Salim reported from Irbil, Iraq. Zakaria Zakaria in Istanbul contributed to this report.